Xcalak is the end of the road: the last Mexican village before Belize, reachable by one long coast road, with a few hundred people and no bank.
Its reef has been a national park since 2000 and is the least visited stretch of the Mesoamerican barrier in Mexico. The wall is close in, the fish are large because nobody has taken them, and most days you will be the only boat on the site. Banco Chinchorro, the atoll fifty kilometres offshore, is reached from here or from Mahahual when the weather holds.
Behind the village, the lagoon and flats are one of the better fly fishing destinations in the Caribbean — permit, bonefish and tarpon, caught and released, mostly wading. Guides are booked through the lodges and the dive centre.
Bring cash and expect the power to go off occasionally. That is the trade for having the reef to yourself.